We are attemping to assess the state of closed circular DNA in solution and to determine whether the facilitated reactions that occur with formaldehyde or other denaturing agents are due to the presence of disrupted base pairs as suggested by others or the formation of such pairs by the denaturing agents themselves. We are also studying the levels of relaxation enzyme at different stages in the cell cycle in mammalian cells separated by centrifugation in Ficoll gradients in zonal rotors. In continuation of our studies of mitochondrial DNA, we are mapping three mammalian DNAs with endonucleases isolated from H. Influenzae. We have found what appears to be a small evolutionarily conserved region in the mitochondrial genome of mammals and plan to examine other phyla for the presence of this DNA fragment.